1. Field of the Invention
The present invention relates to the video endoscope apparatus to observe the inside of body cavities, such as a stomach, intestines and a bronchus.
2. Prior Art
Irradiation of light at a specific wavelength excites living tissue, which causes living tissue to emit fluorescence. Further, intensity of fluorescence emitted from abnormal living tissue that is suffering from a lesion such as a tumor or cancer is smaller than that emitted from normal living tissue. Such a phenomenon also occurs in subcutaneous living tissue under a body cavity wall.
Japanese unexamined patent publication No. P2000-023903A discloses a video endoscope apparatus that finds abnormality of subcutaneous living tissue under a body cavity wall through the use of the phenomenon. A video endoscope apparatus of such a type displays a normal observation image and a special observation image on a monitor. The normal observation image is a color image of a body cavity wall. The special observation image shows an affected area in a predetermined color (for example, red) on a monochromatic (black and white) image of a body cavity wall.
For displaying the normal observation image on the monitor, the video endoscope apparatus irradiates a body cavity wall with white light through the video endoscope and takes color pictures of the body cavity wall with using the video endoscope to generate color image data of the normal observation image. On the other hand, when the video endoscope apparatus operates to display the special observation image on the monitor, the apparatus alternatively emits visible light (reference light) within a predetermined narrow wavelength range and excitation light to excite living tissue. The video endoscope apparatus specifies positions of pixels that should be displayed as affected areas by comparing fluorescent image data that is acquired during the irradiation of the excitation light and reference image data that is acquired during the illumination of the reference light. Then the video endoscope apparatus generates color image data for displaying a monochromatic (black and white) image based on the reference image data.
Since the reference light has the narrow wavelength range, the color image data generated based on the reference image data shows a monochromatic image not a full color image. In this process, the reference image data is converted into a color image format such as RGB component image data.
After that, the video endoscope apparatus converts the color of the specified pixels in the color image data into red, thereby image data of a special observation image is generated. The generated special observation image is two color image data where the red parts exist on the monochromatic (black and white) background.
In the meantime, recently, it is required to observe an image of the body cavity wall (the background) in the special observation image as a full color image instead of a monochromatic image. In order to satisfy the requirement, the pixels representing affected areas may be displayed on the normal observation image.
However, since the reference image data and the fluorescent image data are required to specify the affected areas and the normal observation image data is required to acquire color image data according to the conventional method, the reference light, the excitation light and the white light must sequentially illuminate the body cavity within one cycle to generate one frame of the special observation image data as color image data. That is, the cycle must be divided into three periods.
As described above, since the monochromatic special observation image data is generated by comparing the reference image data and the fluorescent image data, the reference light and the excitation light sequentially illuminate the body cavity within the cycle. That is, the cycle is divided into two periods. Since the cycle for one frame is fixed, accumulation time of the image sensor for each light becomes insufficient when the cycle is divided into three periods.
Accordingly, the brightness values of the normal observation image data, the reference image data and the fluorescent image data tend to lower as a whole.
Further, the lowered brightness values cause an error in comparison of the reference image data and the fluorescent image data, which raises the problem of the disparity between the actual affected areas and the displayed affected areas.